Nitrogen isotope tracing of dissolved inorganic nitrogen behaviour in tidal estuaries

Concentration versus salinity diagrams are the most widely used method to derive estuarine sources and sinks of nitrogen. This method can not distinguish conservative mixing due to a lack of activity, from that due to an approximate balance of sources and sinks. The combined study of concentration and nitrogen isotopic signatures may provide this information because balanced sources and sinks do affect the isotopic composition of dissolved inorganic nitrogen. This is illustrated with three cases of European tidal estuaries: the Scheldt (Belgium/The Netherlands), the Thames (U.K.) and the Loire Estuary (France). Concentration versus salinity diagrams for nitrate showed conservative mixing for the Thames and Loire estuaries and nitrate generation in the Scheldt Estuary. The delta N-15 of nitrate versus salinity diagram confirmed the conservative behaviour for the Thames Estuary and the non-conservative behaviour for the Scheldt Estuary, but revealed active nitrate turnover in the Loire Estuary. Concentration and delta N-15 of ammonium versus salinity diagrams consistently showed non-conservative mixing as a result of ammonium consumption. (C) 2001 Academic Press.